This application relates to novel composites having high surface area, variable porosity and void volume, good conductivity, and displaying chemical stability in corrosive environments. The composites have mechanical and structural integrity and can be prepared in virtually an endless variety of shapes. For the sake of simplicity and clarity of exposition, the composites which are our invention will be discussed from the aspect of their use as electrode materials. However important this particular application may be, it needs to be stressed at the outset that the claimed composites have significant utility outside the field of electrochemistry, but such uses are omitted here only for convenience.
Carbon based electrodes are currently used in many high energy density and/or high power density applications, such as Li/SOCl.sub.2 batteries, liquid double layer capacitors, and fuel cells. The maximum energy and power densities obtainable from these devices depend upon various physicochemical rate phenomena occurring at the electrode-electrolyte interface. For example, in the case of high energy density lithium/thionyl chloride batteries, deactivation of the carbon cathode limits operation of the battery at high (&gt;10 mA/cm.sup.2) discharge rates. Since deactivation arises from the preferential precipitation of solid reaction products at the exterior of the cathode, thereby blocking its interior from participating in the reaction, the power density of the battery during discharge is limited by the porosity, the void volume, and the active or accessible surface area of the carbon cathode.
As previously stated, the low solubility of cell reaction products at the cathode severely limits operation at high discharge rates when precipitates form at the exterior surface of the cathode, blocking its interior surface area from hosting products of the cell reaction. When the cathode becomes blocked, the interfacial electrochemical reaction of the anode becomes limited by the dissolution rate of the reaction products into the electrolyte, which in turn is controlled by the precipitation rate at the cathode. Attempts to improve the fabrication and design of the carbon cathode has had limited success. Much of this activity has involved the addition of metallic elements such as copper to the carbon or the coating of the cathodes with transition metal phthalocyanines. Other efforts have utilized various carbon pretreatment procedures or different types of carbon blacks with various physical properties. However, past attempts appear not to address the intrinsic problem associated with carbon blacks, viz., the inaccessibility of small pores within the microstructure of the material and the existence of low void volumes in the outermost layers of the carbon. To provide high power density cathodes what is needed are materials which are flexible, have high specific surface areas, have varying and adjustable porosities and void volumes to accommodate reaction products as precipitates without significant loss of surface area, and which are corrosion resistant.
In liquid double layer capacitors the energy density increases with increased active surface area of the electrode presented to the electrolyte. On the other hand, the power density is controlled and limited by slow diffusion of electrolyte through the microporous electrode material. The combined energy and power density of these capacitors is the resultant of increased diffusion processes, which prefer large pores and high void volumes, and higher levels of specific surface area, which require small pore sizes and low void volumes. To date the requirements of large pores/high void volume and high surface area tend to be mutually exclusive. Consequently, since increased energy density involves increased surface area and increased porosity, power dense devices become more and more limited by diffusion processes as the surface area of the electrode is increased.
In fuel cells, an effective electrode material should exhibit high catalytic activity and high electrical conductivity to minimize joule losses within the device. The electrode should be highly porous to provide free access to both the gases and the electrolytes. The optimum pore size distribution of the electrode material is a compromise between several factors. For high strength, low porosity and small pores are desirable. For low polarization, large pores with maximum internal surface area are more desirable. Electrodes also contain metals such as platinum, nickel, and so forth, which are good catalysts for fuel oxidation and oxidant reduction. The catalytic activity depends on the active surface area of electrode as well as the contacting of the electrode with reactants consisting of fuel and electrolyte. For this reason, controlled wetting of the electrode poses one of the more severe design limitations confronting the device in order to provide optimal contacting at the gasliquid-catalyst interface in the absence of weeping, bubbling, and flooding.
Carbon is an especially attractive electrode material, and high surface area carbon electrodes typically are fabricated with carbon black. However, a major difficulty in fabricating and utilizing high surface area carbon electrodes has been in physically supporting the carbon. Carbon black usually is used in the powdered form which cannot be easily supported unless poly(tetrafluoroethylene) (PTFE) or other types of binders are used. Our radically different approach has been to combine dissimilar and normally incompatible materials to form a physically stable composite structure which exhibits properties intermediate to the constituent materials. In the context of carbon electrodes, the resulting materials have a high surface area, variable porosity and variable void volume, are structurally stable, and can be fabricated in a virtually endless variety of shapes and sizes. More particularly, high surface area carbon fibers and highly conductive metal fibers have been combined in an interwoven sinter-locked network or grid which is structurally stable. The resultant high surface area and conductive composite allows high accessibility to gases and electrolytes while providing adjustable porosities and void volumes. Interlocked networks of thin fibers can be bonded to metallic backings to provide flexible electrode structures which can be readily assembled into devices even when one of the components is relatively brittle.
A generic approach to high surface area has been to disperse carbon blacks in an organic resin which serves as a mechanical framework. Solomon in U.S. Pat. No. 4,500,647 exemplifies one approach by using a matrix of carbon particles within an unsintered (i.e., unfused) network of carbon black-filamentary PTFE. The use of PTFE as a matrix for carbon particles has been investigated extensively. However, the addition of PTFE reduces the electrical conductivity of the cathode active layer (Solomon et. al. in U.S. Pat. Nos. 4,500,647, 4,518,705, 4,456,521) and the cost of using PTFE has led others to seek alternative means of holding the carbon black together (Aubrey D. Smith, National Technical Information Service Technical Note, Report Date--February, 1986, 1 page, NTN86-0166).
A somewhat different approach employs carbon particles in a carbonized matrix. For example, Christner et. al. (U.S. Pat. No. 4,115,528) prepare a porous carbon sheet by coating carbon fibers with furfuryl alcohol and a catalyst effective for its polymerization. The mixture was formed into a mat, heated to effect polymerization, and the resin then was further heated to carbonize the resin. The patentee in U.S. Pat. No. 4,506,028 dispersed carbon fibers in an organic binder containing organic poreforming granules, then heated the mix to carbonized the binder and volatilize the poreformers. In both of the foregoing the carbonized matrix supplies structure (i.e., rigidity and mechanical strength), whereas in our composites structure is afforded by a grid of fused metal fibers.
Zuckerbrod et. al. in U.S. Pat. No. 4,448,856 describes an electrode with a layer of paste consisting of carbon particles, stainless steel particles, a fluorinated polymeric binder, and a catalyst for decomposition of peroxides. It is noteworthy that such a paste must contain at least 20 weight percent stainless steel relative to carbon particles. Finally, Watanabe and coworkers, [J. Electrochem. Soc: Electrochemical Science and Technology, 134, 1318 (1987)] used polyethylene glycol as a binder for carbon blacks, then pressed a film of the resulting material on nickel wire for use as a cathode in a lithium cell. It may be mentioned in passing that electrochemical electrodes have been described in U.S. Pat. No. 3,905,831 consisting of a pile fabric where a portion of the yarn is metallic. The patentees mention that the metal fibers in the yarn may be bonded, as by sintering, brazing or welding.